the "Qt 6.3 Static Libraries" are only needed IIF you're developing or compiling something that requires them eg. any of the Vee-One LV2 plugins...
if not doing anything of the kind then you don't need them :)
otoh. are you sure you have only installed the ppa:rncbc/apps-focal ?
from the OP it seems you added the ppa:rncbc/apps-jammy as being the only one that probably asks for qt6-qpa-plugins as a dependency :/
the "Qt 6.3 Static Libraries" are only needed IIF you're developing or compiling something that requires them eg. any of the Vee-One LV2 plugins...
if not doing anything of the kind then you don't need them :)
otoh. are you sure you have only installed the ppa:rncbc/apps-focal ?
from the OP it seems you added the ppa:rncbc/apps-jammy as being the only one that probably asks for qt6-qpa-plugins as a dependency :/
cheers